Aluminum alloy



Patented Dec. 15, 1936 Joseph A. Nook, Jr., Tarentum, and Hertha R. Freche, New Kensington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa, acorporation of Pennsylvania No Drawing. Application October Serial N0. 750,016

8 Claims. (o its-'42) This invention relates to aluminum base alloys, especially those containing from about 2 to 12 per cent copper.

Aluminum base alloys containing copper as the predominant alloying constituent have been widely used because of their favorable fabricating characteristics and physical properties. These alloys have also been found to be susceptible to considerable improvement in strength through thermal treatment comprising holding thealloy at an elevated temperature for a sufficient length of time to obtain substantial solution of copper within the limits of its solid solubility in aluminum,.rapidly cooling the alloy to ordinary temperatures and then allowing it to age harden at ordinary or slightly elevated temperatures. The rate of effecting solution at high temperatures and the length of time required to age harden the alloy vary with the physical condition of the alloy, whether it is in cast or wrought form, and with the composition of the alloy, that is, the character and amount of other ingredients than copper that may be present. For commercial purposes it is particularly desirable to control the rate and consequently the length of time needed to heat treat an articlein order to effect economy in operation.

Aluminum base alloys containing 2 to 12 per cent copper and 0.005 to 0.1 per cent tin have 30 been found to possess exceptionally favorable physical properties when heat treated and aged.

To obtain the optimum combination of properties attainable, however, the alloy must be practically free from any magnesium' impurity, that is, a

35 maximum of only 0.01 per cent of this element can be tolerated. In commercial practice a maximum of 0.005 is usually preferred. This requirement has heretofore necessitated the use of high purity aluminum and other alloying ingredients as well 40 as the exercise of extreme care in cleaning the melting pot or furnace hearth prior to melting the alloy therein. Exceedingly small amounts of magnesium on the order of 0.01 to 0.02 per cent have been found to exert a marked deleterious influence upon the strength and corrosion resistance of the above described alloy in'the heat treated and aged condition.

One of the objects of our invention is to overcome the harmful effect of magnesium in aluminum-copper-tin base alloys without disturbing the responsiveness of the same to heat treatment or diminishing the resultant physical properties.

A further object is to accomplish the foregoing purpose by a convenient means that is readily adaptable to prevailing plant practices.

We have discovered that the addition of from about 0.05 to 0.15 per cent of cadmium to an aluminum base alloy containing from about 2 to 12 per cent copper, 0.005 to 0.1 per cent tin and 0.005 to 0.03 per cent magnesium substantially overcomes the adverse effect of magnesium upon the properties of the heat treated alloy.

To obtain the best results the proportion of cadmium to magnesium should be in the ratio or 5 to 1 by weight, respectively. The amount ofmagnesium impurity present will vary with the metal used and the cleanliness of the melting or holding pot. When the precise amount of the impurity is not known enough cadmium should be added to counteract the magnesium believed to be present. An excess of cadmium over the optimum. ratio is not detrimental to thealloy and it is preferable to allowing a deficiency of cadmium with respect to the quantity of magnesium to be counteracted.

While we offer no explanation of the remark able behavior of cadmium, we have found it to be the only element of a related group of metals 4 that functions satisfactorily as a counteractor of magnesium in the herein described type of alloy.

The harmful effect of magnesium on heat treat- 2 ed aluminum base alloys containing copper and tin is particularly manifested by a decrease in yield strength. An alloy, for example, containing about 4.4 per cent copper, 0.8 per cent manganese, 0.8 per cent silicon and 0.05 per cent tin, the balance being aluminum, with less than 0.005 per cent magnesium impurity, was heat treated at about 510 C., quenched and aged at 160 C. This alloy had a tensile strength of 63,500 pounds per square inch, and a yield strength of 53,000 pounds per square inch. The samealloy with 0.025 per cent magnesium present when heat treated under thessame conditions had a tensile strength of 61.100 pounds per square inch and a yield strength of only 43,300 pounds per square inch. This example well illustrates the profound influence of an almost negligible quantity of an impurity and why extra precautions have been necessary in the preparation of such alloys to avoid contamination with magnesium.

The aforesaid deleterious effect of magnesium may be overcome through the addition of a small amount of cadmium. 1 An aluminum base alloycontainingabout 4.4 per cent copper, 0.8 per cent manganese, 0.8 per cent silicon, 0.05 per cent ,tin, 0.02 per cent magnesium and 0.1 per cent -cadmium, the balance being aluminum, was heat treated at 510 C., quenched, and aged at 160 C. as were the alloys described hereinabove. In this condition it had a tensile strength of 65,000 pounds per square inch and a yield strength of 54,200 pounds per square inch. These properties compare very favorably with those of the other alloys mentioned above, especially the alloy containing 0.025 per cent magnesium. It is to be further observed that the strength of ourimproved alloy containing cadmium was even above that of the normal alloy containing virtually no magnesium impurity, showing that the harmful influence of magnesium was completely counteracted.

, Comparative corrosion tests have also shown nie presence of 0.025 per cent magnesium caused to in the art as artificial aging and is usually a markedincrease in the losses of the second alloy, the tensile strength being diminished by 13 .per cent from the original value and the elongation dropping '70 per cent from what it was in the uncorroded alloy. In the third alloy to which 0.1 per cent cadmium had been added, the tensile strength only decreased 3 per cent during the corrosion period and the elongation fell 13 per cent. An extension of the test over a period of 12 weeks revealed the same diiferences between the alloys. These test data' in: dicate the injurious influence of a very small amount of a particular impurity, and further, that such effect may be readily overcome and the alloy actually improved by the addition of less than 0.15 per cent of a second element. The use of cadmium in this manner thus permits employment of stock metal of a lower purity than has heretofore been possible.

Although the beneficial influence of cadmium is exhibited over the entire range of 2 to 12 per cent copper and 0.005 to 0.1 per cent tin, we have found that the addition of cadmium is particularly effective within the range of 2 to 6 per cent copper which correspondsto the copper content found in most wrought and many cast aluminum-copper base alloys. Also in our preferred practice we employ from about 0.02 to 0.07 per cent tin. The amount of cadmium necessary to counteract the magnesium present generally varies between about 0.05 and 0.15 per cent. when silicon and manganese are present, we prefer to limit their range to about 0.1 to 1 per cent each.

, The alloys herein described are susceptible to fabrication in the manner generally practiced in the art of making and shaping aluminum base alloys. They may, furthermore, be subjected to the usual thermal treatment employed to improve the strength of aluminum base alloys. This treatment generally consists in heating the alloys at an elevated temperature, above about 475 C., and rapidly cooling to ordinary or slightly elevated temperatures. This may be followed by an aging at room temperature or at any temperature up to about 200 C. The aging above room temperature is generally referred in the appended claims denotes metal of the purity, exclusive-of the magnesium impurity as herein defined, that is commercially available.

other alloying ingredients than those hereinbefore disclosed may be utilized in making heat treated alloys containing copper and tin so long as the function of these two elements is not substantially disturbed.

We claim:

1. A heat treated and artificially aged aluminum base alloy containing from about 2 to 12 per cent copper, 0.005 to 0.1 per cent tin, 0.005 to 0.03-- per cent magnesium and 0.01 to 0.15 per cent cadmium, the balance being aluminum.

2. A heat treated and artificially agechaluminum base alloy containing from about 2 to' 12 per cent copper, 0.005 to 0.1 per cent tin,

0.01 to 0.15 per cent cadmium and 0.005 to 0.03 percent magnesium as an impurity, said alloy being characterized by the fact that its tensile strength, yield strength, corrosion resistance and responsiveness to heat treatmentare considerably improved over the corresponding properties in an aluminum-base alloy containing the same amount of copper, tin, and magnesium but no cadmium.

5. A heat treated and artificially aged aluminum base alloy containing from about 2 to 6 per cent copper, 0.1 to l per cent manganese, 0.1 to l per cent silicon, 0.005 to 0.1 per cent tin, 0.005 to 0.03 per cent magnesium and 0.01

to 0.15 per cent cadmium, the balance being 0.15 per cent cadmium, the balance being aluminum and the ratio of. cadmium to magesium being at least 5 to 1.

8. A heat treated and artificially aged aluminum base alloy containing from about 2 to 12 per cent copper, 0.005 to 0.1 per cent tin, 0.005 to 0.03 per cent magnesium, and 0.01 to 0.15 per cent cadmium, the ratio of the cadmium to the magnesium being at least 5 to 1, said alloy being characterized by the fact that its tensile strength; yield strength, corrosion resistance, and responsiveness to heat treatment are considerably improved over the corresponding properties in an aluminum base alloy containing the same amount of copper, tin, and

magnesium, but no cadmium.

JOSEPH A. NOCK, JR. HERTHA R. FRECI-IE. 

